1. Field of the Invention
The present invention relates in general to an improvement of silicon fusion bonding of silicon substrates for preparation of silicon structure, and more particularly to a method and apparatus for silicon fusion bonding of the silicon substrates which fills up micro-gaps of a junction interface between the silicon substrates and improves junction strength of the junction interface by growing of wet-type oxide layers at the micro-gaps of the junction interface through a stabilizing process for adsorbing a great quantity of oxidant and a high temperature heat-treating process in the presence of oxidant after completion of the pre-process of silicon substrates.
2. Description of the Prior Art
In recent years, there has been proposed silicon fusion bonding method for forming silicon on insulator (SOI) structure used for a three dimensional semiconductor element or micro-mechanical structure used for a silicon sensor and actively studied for improving such a silicon fusion bonding method for preparation of the silicon structures.
A conventional silicon fusion bonding method for junction or bonding of the silicon substrates will be described as follows.
First, the silicon substrates to be bonded to each other are washed according to a standard wafer washing such as RCA washing in order to clean their surfaces and immersed in an alkaline solution having OH ions, thereby causing a hydrophilic layer having a great quantity of OH ions to be formed thereon. The result silicon substrates or the silicon wafers having the hydrophilic layer are then dried and faced to each other such that their surfaces face to each other and this permits the silicon wafers to be primarily bonded to each other by OH ion bond, so called hydrogen bond.
Thereafter, the bonded wafers which have been subjected to the aforementioned processes are heat-treated at a high temperature higher than about 1,000.degree. C. for about 60 minutes so as to intend to improve the junction strength of the junction interface of the result wafers and to remove the non-bonded parts from the junction interface.
In the aforementioned known silicon fusion bonding method, it is noted that the junction strength of the silicon wafers when they are primarily bonded to each other by the OH ion bond is about 5 to 10 kg/cm.sup.2, which is considered as relatively lower strength, and there are occasionally non-bonded parts at the junction interface of the result bonded wafers. However, these problems, or the relatively lower junction strength and presence of non-bonded parts, can be conventionally overcome by the continued heat-treating process. That is, the heat-treating process carried out at the high temperature results in elastic/plastic deformation of the silicon substrates at the junction interface and/or solid state diffusion of silicon or oxygen atoms and this makes the initial Si-OH-OH-Si type junction be transformed to Si-Si type junction and, as a result, improves the junction strength of the junction interface to at least 150 kg/cm.sup.2 and removes the non-bonded parts from the junction interface.
However, in order to cause the elastic/plastic deformation of the silicon substrates and the solid state diffusion of the silicon or oxygen atoms, the primarily bonded silicon wafers are necessarily subjected to the heat-treating process carried out at the high temperature higher than 1,000.degree. C. for at least 60 minutes and, in this regard, the doping profile previously formed in the silicon wafer and/or on the silicon wafer surface is deformed. As a result, the disadvantage of the known silicon fusion bonding is resided in that the deformation of the doping profile causes a process error to occur in the continued preparation processes, for example, thinning process using electrochemical etching stop for providing silicon membrane structure for the silicon wafers.